On Jul 7, 1:57 pm, Wolfi wrote:
Am 07.07.07 13.06 schrieb Golf:





On Jul 7, 11:44 am, "David" wrote:
"Golf" wrote in message

groups.com...

This may seem to be a stupid question to many of you, but
here it goes
- I checked a horizontal output transistor with my fluke
(diode
check). The transistor is an NTE2353. It has a built in
damper diode.
Since I don't know how this diode is fixed in the
transistor, I'm not
sure if I'm getting accurate results. I get normal voltage
drop
between the base and collector. I read dead short between
base and
emitter (both ways). Regardless of having a damper diode,
I shouldn't
get this reading on a good transistor, right? I just want
to be sure
before buying a repalcement. Thanks group.
Many horizontal output transistors have a built in B-E
resistor of 30 ohms or so. A dead short means it is bad.

David- Hide quoted text -

- Show quoted text -

Thanks. I did check and I read 36 ohms between B and E. But as I said,
doing a diode check, I read dead short both directions. Shouldn't a
resistance reading only be able to be taken with common lead on
emitter if transistor (NPN) is good?

The damper diode is connected between C and E (which usually is ground), with
its anode connected to E and its cathode to the transistors C.
So if the diode is still good and your transistor's C-E path is not shorted,
then in diode test mode you should only get a reading with (+) of your DMM
connected to the transistor's E and (-) touching its C.
In respect of the 36Ohm base resistor, this value is too low for your DMM's
diode test to tell you anything reliable. But still reading the 36 Ohm means
your B-E path is not shorted, which is rather unlikely anyway. They usually
short C-E, or quite occasionally C-B.

From what you wrote so far, I'd conclude that the transistor is still OK.
But you could do a quick test with a 12V source. f.e. car battery, a 10W light
bulb (dome- or number plate light), a few connector wires with crocodile clips
and a resistor some 220 Ohm to limit the base current.
If you can turn on and off the light by touching releasing the base with the
resistor, the LOT should be fine. Also reverse polarity on C-E to test the
diode. In this case the bulb should be on constantly.- Hide quoted text -

- Show quoted text -

I did a similiar test. Only without light bulb. Properly biased the
trans. It worked. Removed the base voltage, and it still conducted
with the same forward voltage across the emitter and collector. My
conclusion, since I can't turn off the transistor by removing base
voltage, it's shot. By the way, I did have to replace another NPN in
the SMPS of this same chassis. The Base and Emitter were shorted. The
Emitter to Collector checked good. Thank you to all that responded. I
wanted to be sure of what I already suspected, just the damper diode
made me question if my test results were accurate. If ya'll agree with
my findings, I am going to go ahead and order the replacement HOT.
Thanks again group.